Field of the Invention
The invention relates to a method for acquiring magnetic resonance data with a magnetic resonance system using a magnetic resonance sequence wherein, in a first partial sequence, magnetic resonance signals are acquired from multiple slices simultaneously. From these magnetic resonance signals, magnetic resonance data for the individual slices are calculated with a reconstruction algorithm. In addition, the invention relates to a magnetic resonance system for implementing such a method.
Description of the Prior Art
In magnetic resonance imaging, it is generally known to detect different slices in a target object from which magnetic resonance data are acquired. This type of magnetic resonance imaging is also called two-dimensional magnetic resonance imaging, because the slices are reproduced in two-dimensional magnetic resonance images. Techniques are known that make it possible to acquire multiple slices simultaneously in order to reduce the repetition time (TR) and the total acquisition time. In particular, these techniques are advantageous when the standard protocol, i.e., without simultaneous acquiring of multiple slices, involves a long repetition time.
In this case, the reduction of the repetition time, which was introduced with the implementation of a simultaneous acquisition of multiple slices, does not result in significant spin saturation effects, and the reduction of the total acquisition time can be achieved with minimal losses in the signal-to-noise ratio (SNR).
Two basic techniques for simultaneous acquiring of multiple slices are known.
In a first of these techniques, described in detail in U.S. Pat. No. 6,614,225, slight temporal offsets between slice-specific excitation and refocusing pulses are used to achieve temporal offsets between the echoes of each individual slice to a single excitation signal. Subsequently, these echoes can be scanned separately so that the resulting magnetic resonance signals can be assigned to the correct slice.
The second known method for simultaneous imaging of multiple slices utilizes modified radio-frequency pulses to excite and refocus the magnetization of several different slices in an actually simultaneous manner. The resulting echoes are also produced simultaneously so that the scanned magnetic resonance signals contain superimposed individual signals of the different slices. Subsequently, the signal of the individual slices can be separated by using position-dependent information of multiple receiving coils, see, for example, the article by D. J. Larkman et al., “Use of multi coil arrays for separation of signal from multiple slices simultaneously exited”, J. Magn. Reson. Imaging 13: 313-317 (2001). Therefore, this involves the use of so-called “parallel imaging”, in which the magnetic resonance signals of several acquisition (receiving) coils are acquired and evaluated. For example, at the same time, it is possible to perform subsampling to reduce the entire acquiring time, wherein it is still possible to determine missing information by the magnetic resonance signals of several coils. In this context, the term subsampling is used for subsampling in “in-plane” direction (for example, the y-direction). For each slice, reference data are also used, which can be determined in the usual manner, i.e., without simultaneous acquiring of several slices, optionally with lower spatial resolution, and which are used to determine the magnetic resonance data of different simultaneously acquired slices.
The GRAPPA technique is a frequently used version of such parallel imaging, see, for example, the article by M. A. Griswold et al., “Generalized autocalibrating partially parallel acquisitions (GRAPPA)”, Magn. Reson. Med. 47 (6): 1202-1210 (2002). The slice GRAPPA technique does not necessarily use a subsampling method, but when acquiring data a complete scan is performed, which, however, involves superimposed individual signals of multiple slices. However, it is known to produce a type of artificially subsampled data set by duplicating during data processing the field of view (FOV), for example, in the y-direction. As a result, the offset image of the individual slices is no longer subject to the aliasing effect within the small FOV.
The so-called CAIPIRINHA method has been proposed as an especially useful and important extension for the basic idea of simultaneous excitation and refocusing. It has been described in an article by F. A. Breuer et al., “Controlled aliasing in parallel imaging results in higher acceleration (CAIPIRINHA) for multi-slice imaging”, Magn. Reson. Med. 53: 684-691 (2005). This method, the magnetic resonance sequence is modified, resulting in a slice-specific, in-plane offset of the image pixels. This modification improves the performance of the reconstruction algorithms, which are used to separate the individual signals of the different slices present in the magnetic resonance signal, for example, the well-known slice GRAPPA algorithm. A variation of this method is used for echo planar imaging (EPI), which is known as “blipped CAIPIRINHA”, see, for example, US 2011/0254548 A1 or the article by K. Setsompop et al., “Blipped-controlled aliasing an parallel imaging for simultaneous multislice echo planar imaging with reduced g-factor penalty”, Magn. Reson. Med. 67: 1210-1224 (2012).
It has been proposed to combine the (“blipped”) CAIPIRINHA method for simultaneous imaging of multiple slices, which uses short gradient pulses, with the diffusion-weighted, readout-segmented echo planar imaging sequence (rs-EPI—readout-segmented echo planar imaging), see, for example, U.S. Pat. No. 7,205,763 or the article by D. A. Porter et al., “High resolution diffusion-weighted imaging using readout-segmented echo-planar imaging, parallel imaging and a two-dimensional navigator-based reacquisition”, Magn. Reson. Med. 62: 468-475 (2009). Examinations, as described, for example, in an article by R. Frost et al., “Reduction of diffusion-weighted readout-segmented EPI scan time using a blipped-CAIPI modification”, Proc. Annual Meeting of ISMRM 2012, abstract 116, show that the combination of these two methods provide a diffusion-weighted imaging technique with improved image quality when compared to standardized single shot echo planar imaging (ss-EPI—single shot EPI), while a short total acquisition time, which is acceptable for routine examinations in medical operations, is achieved.
In this context, the use of real time feedback based on navigators also has been proposed, see, for example, the above-mentioned article by D. A. Porter et al., or U.S. Pat. No. 7,417,427 B2. The acquisition of navigators forms a second partial sequence of the magnetic resonance sequence, which shares the excitation signal emitted in the first partial sequence for acquiring the actual magnetic resonance data, and which uses a new refocusing pulse and a new readout time period. Usually, a navigator feedback is used in connection with the rs-EPI-sequence or other diffusion-weighted magnetic resonance sequences to identify and newly measure readout segments with strong, movement-induced phase errors, if they cannot reliably be corrected by a correction, for example, a phase correction, taking into consideration the two-dimensional navigators. In this way, bulk movements of the entire acquiring area are prevented from greatly distorting the diffusion determination. In the context of brain imaging the strong phase errors have a highly non-linear performance resulting from a brain non-uniformity due to pulsating cerebrospinal fluid (CSF). This makes it possible to identify the readout segments affected by measuring the breadth of magnetic resonance distribution in k-space, even when subsampled magnetic resonance data sets are recorded using parallel imaging, for example, with the use of GRAPPA technique. This simple possibility for identifying affected readout segments is especially useful for real time measurements, because the magnetic resonance data (navigator data) concerning the navigators can be evaluated without the computational expense of a complete image reconstruction, which usually requires a multitude of steps, especially rebinning for non-equidistant kx scan points and using a parallel imaging algorithm.
However, simultaneous imaging of several slices, as discussed above, involves a problem, because the raw data, i.e., the navigator magnetic resonance signals, also contain the superimposed individual signals of multiple slices in the navigator. Therefore, it is less easy to identify the slice-specific phase errors, even when the magnetic resonance signals of different reception coils are evaluated separately, which provides a certain spatial dependency of the magnetic resonance signals. In principle, it would be possible to apply the reconstruction algorithm, for example, a slice GRAPPA algorithm, also to the navigator data, but this would have to take place in real time in order to control, by means of the navigator data, the acquisition process and especially the remeasuring processes. This real time application of the reconstruction algorithm would involve extremely high computational demands because, for most reconstruction algorithms, reference data are required that have to be processed, and the extraction of slice-specific navigator data, as well as the processing of reference data, would have to take place in real time.
For example, the application of the slice GRAPPA algorithm (“slice GRAPPA”) requires a set of reference data concerning the central readout segment, which is usually acquired by using a standardized multi-slice acquisition technique without simultaneous imaging. A basic problem of this additional acquisition of reference data involves additional acquisition time, which is required for acquiring the reference data.